Resistance welding of galvanized steel

ABSTRACT

A coating for improved resistance welding of galvanized steel parts or sheets comprises a binder, and a metal phosphide pigment, preferably a ferrophosphorus pigment, having a particle size of from about 0.1 to about 30 microns. The pigment can also include up to about 40% by weight of an additional metal such as tin, aluminum or lead, which can be combined with the metal phosphide in pigment form or deposited onto the surface of the particles. The coating can be applied to either the faying or non-faying surfaces of the galvanized steel, or to the resistance welding electrode, and may be removed, if desired, after the welding operation. 
     The advantages of the present invention include a significant reduction in welding current and an increase in the weldability lobe width, a restoration of the dynamic resistance beta peak, and an increase in electrode life.

BACKGROUND OF THE INVENTION

The present invention relates to an improvement in the resistancewelding of galvanized, i.e. zinc or zinc alloy coated, steel parts orsheets which is achieved by coating either the alvanized surface to bewelded or the welding electrode with a resin binder containing a metalphosphide pigment, and preferably a ferrophosphorus pigment. The weldingimprovements realized by practicing the present invention are improvedweldability lobes and dynamic resistance curves for better weldingcontrol for resistance welding systems as well as increased electrodelife.

The use of galvanized steel sheets in the automotive industry has becomeincreasingly popular in recent years due to the increase in concern forcorrosion protection for automobile body panels. Corrosion problems areparticularly severe in environments where salt is used for preventingthe icing of snow on highway roads. Although efforts have been made toenhance the corrosion-resistance of steel sheets, such as by usingvarious chemical conversion treatments and paint coatings, the corrosionprotection method of choice currently is galvanized steel, with thegalvanized coating formed by either hotdipping or electrodeposition.

For zinc or zinc alloy coated sheet steels to successfully substitutefor uncoated sheet steels, they must exhibit acceptable formability andweldability characteristics. As a general rule, coated steels have notdemonstrated properties as good as their uncoated counterparts. Users ofthese products are continually looking for new coated sheet steels whichprovide the advantages of a coated steel, but have weldability andformability characteristics similar to uncoated steels.

The most common method of joining steel sheets (particularly in theautomotive and appliance industries) is ressstance spot welding.Resistance spot welding is ideally suited for joining thin sheetmaterials and is well adapted to mass production industries. Inaddition, operating costs for this process are relatively low.Resistance spot welding has been used with uncoated steels quitesuccessfully since the 1930's.

Resistance spot welding is used to form joints between two materials.The process uses a set of electrodes to apply pressure to the weld area,to maintain the components in position, and to pass current through theweld. As the current flows, joule heating of the substrate occurs. Duein part to the cooling effects of the electrodes, a molten nuggeteventually develops at the weld centerline or faying surface. Oncooling, this nugget resolidifies and effects a joining between the twomaterials.

As mentioned, resistance spot welding of uncoated steels hashistorically been quite successful. However, the resistance spotweldability of coated sheet steels has not been as successful. Theproblems can be best seen by reference to some typical measures of spotweldability.

The weldability lobe is defined as the range of welding conditions (weldcurrent and weld time) over which weld nuggets of an adequate size canbe formed. This, in effect, defines a "window" of acceptable weldingconditions. When practical, weld nugget sizes during lobe testing areestimated with a destructive test known as the peel test. This testconsists of welding two 11/4-inch by 4-inch samples at two points, anddestructively pulling apart the second of the welds. The weld nuggetwill usually adhere to one of the two sheets as a weld "button", and thesize of this weld button can be measured with a set of calipers. Theweld button size is usually considered a good measure of the nuggetsize. The limits of the weldability lobe are defined by the weldingconditions which produce a minimum weld size on one side, and expulsionon the other (expulsion occurs when liquid metal is expelled from theweld during welding). A line representing a nominal button size (partway between the minimum and expulsion) is also often included.

The weldability lobes are characterized by lobe position, lobe width andthe position of the nominal button line. See, generally, D. W.Dickinson, Welding in the Automotive Industry, Report SG 8-15 of theCommittee of Sheet Steel Producers, the American Iron and SteelInstitute. The lobe position is defined as the average welding currentof the lobe. Though lobe position is not considered to be a criticalweldability parameter, higher welding currents do result in higherenergy costs, as well as a decrease in electrode life. More significantis the width of the weldability lobe defined as the difference inwelding currents between minimum button and expulsion at a particularwelding time. This is a measure of a materials' "flexibility" duringspot welding. The position of the nominal button line, althoughconsidered of lesser importance, is also a measure of a materials'flexibility during spot welding. A central position for this lineindicates a button size with adequate current range to both higher andlower currents.

Dynamic resistance is used as a measure of weld quality and is definedas the resistance of the weld across the electrodes (as a function oftime) during welding. The dynamic resistance has been correlated to welddevelopment in uncoated steels, and successfully used as an input signalfor feedback control. Unfortunately, the results for zinc or zinc alloycoated steels have not been as good. In particular, feedback systemshave been largely unsuccessful in controlling weld development in suchcoated steels which exhibit a featureless resistivity trace or curve.The dynamic resistance trace for uncoated steel, in contrast, exhibits acharacteristic "beta peak", followed by a resistance drop. It is thepresence of this "beta peak" which makes resistive feedback controlpossible. See Dickinson, supra.

When resistance welding uncoated steels, a single set of copper weldingelectrodes can be expected to make approximately 50,000 welds. Whenwelding galvanized steels, however, the electrode life is reduced toabout 1000--2000 welds or less. Since the production line must bestopped each time an electrode is replaced, at a considerable expense tothe user, the relatively limited electrode life for galvanized steelsrepresents a significant economic disadvantage.

The use of ferrophosphorus pigment for both improved corrosionprotection and weldability has been suggested in the prior art. Forinstance, U.S. Pat. No. 3,884,705, issued May 20, 1975 and U.S. Pat. No.4,119,763, issued Oct. 10, 1978, both disclose the use of coatingscontaining ferrophosphorus and zinc pigments, and a non-metalliccorrosion inhibitor such as zinc chromate, as a replacement forzinc-rich coatings. These coatings also contain a non-metallic corrosioninhibitor such as zinc chromate. As contemplated in these patents, theferrophosphorus pigment-containing coating is applied to bare steelpanels rather than to galvanized sheets. The ferrophosphorus pigmentused in such applications is commercially available from the OccidentalChemical Corporation under the trademark Ferrophos® pigment.

A ferrophosphorus pigment dispersed in a resin to bind adjacent steelplates to form a vibration-damping composite suitable for resistancewelding is disclosed in Japanese Patent Application No. 61-41540,published on Feb. 27, 1986.

The use of a coating comprising a resin, ferrophosphorus powder and micapowder applied to a steel sheet having a layer of fused aluminum or analuminum/zinc alloy is disclosed in Japanese Patent Application No.591456884, published Aug. 22, 1984. The steel sheet described in thisreference can be subjected to chemical conversion, and is furtherdescribed as having excellent weldability, processability and corrosionand heat resistance.

The use of an iron layer containing eess than about 0.5 weight percentphosphorus applied to a zinc/iron or zinc/nickel alloy electroplatedsteel sheet for improved surface properties is described by Honjo et al.in Internal Journal of Materials and Product Technology, Vol. 1, No. 1,pp. 83-114 (1986).

It will be appreciated by those skilled in the art that a continuingneed exists for steel sheets which possess the durability and corrosionresistance of galvanized sheets but also possess the weldabilityadvantages of bare steel.

SUMMARY OF THE INVENTION

In accordance with the present invention, a zinc or zinc alloy coatedsteel sheet or part with improved resistance welding characteristics hasa top caating of a binder and a pigment consisting essentially of atleast one metal phosphide selected from the group consisting ofphosphides of iron, nickel, cobalt, tin, copper, titanium, manganese,molybdenum, tungsten, vanadium, tantalum and mixtures thereof.Preferably, the metal phosphide is ferropoosphorus pigment having arange of particle sizes of from about 0.1 to about 30 microns, and whichis present in the coating composition in amounts of from about 30% toabout 90% by weight of non-volatile components.

The metal phosphide coating can also be applied to a copper resistanceelectrode for improved weldability. An additional metal can be appliedto the electrode surface prior to coating the electrode with the metalphosphide, the additional metal being selected from the group consistingof iron, nickel, cobalt, silver, manganese, vanadium, molybdenum andgold.

The pigment can also include up to about 40% by weight of a metaladditive selected from the group consisting of tin, aluminum or lead.This metal additive can be physically combined with the ferrophos inpigment form or deposited onto the surface of the ferrophosphoruspigment. These additive metals are used to increase the electrode life.

The use of a coating containing a ferrophosphorus pigment applied to thefaying surfaces of a galvanized steel sheet or part results in asubstantial decrease in the welding current and an increase in theweldability lobe width as compared to galvanized steel. If such acoating is applied to the non-faying surfaces, or to the resistancewelding electrode, an increase in electrode life results. In addition,the use of a coating containing ferrophosphorus pigment results in arestoration of the dynamic resistance beta peak to the dynamicresistance trace.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The steel sheets or formed parts which are used in the present inventioncontain a thin layer of zinc metal or a zinc alloy which is directcontact with the steel surface. Typically, the zinc layer has athickness of about 0.5 mils. The steel substrate itself is generallyabout 30 mils thick. Thin steel sheets of this type are used extensivelyin the automotive and appliance industries for forming auto andappliance bodies. The zinc or zinc alloy coating or layer is typicallyapplied to the steel sheet using well-known techniques such as hot-dipgalvanizing, where the sheet is contacted with molten zinc, orelectrogalvanizing, where zinc or a zinc alloy coating is applied to thesubstrate by electrodeposition. This invention, however, does notcontemplate the further treatment of steel sheets having a layer ofhighly electrically resistant material such as Zincrometal. A steelsheet or part having this coating is characterized by a high surfaceresistance which results in the absence of a dynamic resistance betapeak.

The metal phosphide pigment of the present invention comprises particleshaving an average size within the range of from about 0.1 to about 30microns. Particles within the desired size ranges are suitably obtainedby pulverizing hhe metal phosphide using conventional techniques.Suitable metal phosphides include phosphides of iron, nickel, cobalt,tin, copper, titanium, manganese, molybdenum, tungsten, vanadium,tantalum, as well as mixtures of these metal phosphides. The preferredmetal phosphide is iron phosphide, which includes various ratios of ironand phosphorus, and particularly ferrophosphorus, which is an ironphosphide compound containing from about 20% to 28% of phosphorus andcorresponding chemically to a mixture of Fe₂ P and FeP. Ferrophosphorusis obtained as a by-product in the commercial manufacture of elementalphosphorus by the electric furnace reduction of phosphate ores, with theiron present in the phosphate ores forming the ferrophosphorus.Ferrophosphorus typically contains impurities, of which silicon andmanganese are the major impurities, typically being present in amountsof up to 5% by weight, and is further characterized as beingelectrically and thermally conductive, brittle, and substantiallyunreactive in water, dilute acidic or alkaline environments. Aparticularly suitable ferrophosphorus pigment is Ferrophos® pigmentwhich is manufactured and sold by the Occidental Chemical Corporation.

A coating composition containing the metal phosphide pigment of thepresent invention may be formulated by admixing the metal phosphideparticles with a suitable binder, also using conventional mixingtechniques. More specifically, when the metal phosphide of the presentinvention is incorporated into a coating formulation, the bindercomponent of the formulation comprises 5% to 96% by weight ofnon-volatile components, and preferably from 10% to 70% by weight of thenon-volatile components. Various binder materials, both organic andinorganic, may be used, the choice of a particular binder beingdependent upon the characteristics which are desired for the particularapplication. Typical binders include various synthetic resins, such asepoxies, chlorinated rubber, polystyrene, polyvinyl acetate resins,silicones, silanes, borates, silicates, acrylics, polyurethanes and thelike. In some applications, it may be desirable to apply a coating whichcan be readily removed after the welding operation. Typical binders ofthis type, i.e. those which are readily removable, include, by way ofillustration, carboxymethyl cellulose, ethyl cellulose, polyvinylalcohol, natural gums, etc. Other suitable binders not specificallydescribed herein will be readily apparent to those skilled in the art.

The metal phosphide pigment can be present in the coating in an amountof from about 4% to about 95% by weight of the total non-volatilecomponents in the coating, with amounts within the range of from about30% to about 90% by weight being preferred. A portion of the metalphosphide particles of the pigment can be replaced by other metals suchas tin, aluminum and lead. These additional metals can be present inamounts of up to about 40% by total weight of the pigment, and willtypically have an average size within the range of from about 0.1 toabout 30 microns. Alternatively, the additional metal can be depositeddirectly onto the surface of the metal phosphide particles usingtechniques which are well-known to those skilled in the art, such as byphysically grinding or blending mixtures of the metal phosphide andadded metal in the desired proportions, or by immersion coating, etc.

Depending upon the particular binder which is selected, the coatingcomposition may also contain suitable solvents, curing agents,suspending agents, plasticizers and the like. The selection of the typeand amounts of these other components will of course depend upon theparticular binder as well as the ultimate characteristics desired forthe coating and its use.

The formulated coating may be applied directly to the substrate usingany available technique such as, for example, spraying, brushing,immersion, flowing or the like. If desirable, an intermediate conversioncoating can be applied to the substrate prior to the application of themetal phosphide-containing coating. Typically, the coating is applied toproduce a film having a thickness within the range of about 0.1 to 10mils, although thicknesses which are outside of this range may also beused to advantage.

In another embodiment, the coating containing the metal phosphide can beapplied to the resistance electrode rather than or in addition toapplication of the coating to the substrate. In this embodiment, theresistance welding electrode can be first coated with a metal selectedfrom the group consisting of iron, nickel, cobalt, tin, copper,titanium, manganese, molybdenum, tungsten, vanadium, tantalum, andmixtures thereof prior to application of the metal phosphide-containingcoating. Suitable methods of applying the coating to the resistanceelectrode include spraying, brushing, contact with an expendable ribboncontaining the metal phosphide, and other methods as will be readilyappreciated by those skilled in the art.

The coating containing the metal phosphide can be applied to either thefaying or the non-faying surfaces of the steel sheet or part, or both asdesired. Application of the coating to only the faying surfaces resultsin improvements in the welding lobe curve and dynamic resistance curve,while application of the coating to the non-faying surfaces results inimprovements in electrode life. The presence of theferrophosphorus-containing coating at the faying surfaces reduces shuntcurrents and, consequently, the temperature of the electrode, increasingelectrode life.

The following specific examples are provided as exemplary of variousembodiments of the present invention, but are not intended to limit thefull scope of the invention as defined by the appended claims. EXAMPLES1-3

Lobe curves were generated using procedures established by Fisher BodySpecification MDS-247 for galvanized steel. Welding conditions were asfollows:

    ______________________________________                                        Welding Electrodes:                                                                          RWMA Class II, 45 degree truncated                                            cone, 0.25 inch face diameter                                  Welding Force: 500 pounds                                                     Weld Times:    11, 14, 16 and 19 cycles                                       Minimum Nugget Size:                                                                         0.16 inch                                                      Nominal Nugget Size:                                                                         0.20 inch                                                      ______________________________________                                    

This test consists of welding two 11/4-inch by 4-inch coupons 0.03inches thick at two locations, and destructively pulling apart thesecond weld. The diameter of the peeled weld nugget was measured todetermine the position of the limit lines which comprise the weldabilitylobe. The orientation of the coupons was such that the coating on thetop coupon was a the electrode-to-sheet interface, and the coating onthe bottom coupon was at the sheet-to-sheet interface.

Dynamic resistance traces were also obtained for welds made on each ofthe materials to help interpret nugget development during spot welding.These curves were also used to characterize each material's suitabilityto feedback control. These were obtained both across the welding tipsand where necessary across the sheets. The resistance weldingcharacteristics of untreated coupons as well as coupons having variouszinc metal or zinc alloy coatings was evaluated. The hot-dippedgalvanized layer was applied in amounts of from about 0.9 to about 1.25oz. of metal per square foot, while the Zincrometal coating was about0.5 mils thick. The types of coupons evaluated were as follows:

    ______________________________________                                        EXAMPLE NO.    COATING                                                        ______________________________________                                        1              None                                                           2              Hot-dipped galvanized                                          3              Zincrometal                                                    ______________________________________                                         The weldability of the coupons of Examples 1--3 was evaluated, and the     results of the evaluation are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                 Lobe     Nominal Location                                                                           Presence of                                             Width    of Lobe      Dynamic Resistance                             Example No.                                                                            (Amps.)  (Amps.)      Beta Peak                                      ______________________________________                                        1        2000     8300         Yes                                                              (central)                                                   2        1600     11200        No                                                               (min.)                                                      3        1800     7200         No                                                               (central)                                                   ______________________________________                                         EXAMPLES 4-6

The metal phosphide-containing coating of the present invention wasevaluated using an epoxy ester binder containing 92% by weight of aferrophosphorus pigment, designated FERROPHOS® HRS 2131 with a meanparticle size of 5 microns, which is manufactured and sold by theOccidental Chemical Corporation. This coating was sprayed onto varioussubstrate materials to a thickness of 1 mil as follows:

    ______________________________________                                        EXAMPLE NO.    SUBSTRATE                                                      ______________________________________                                        4              Bare Steel                                                     5              Hot-dipped galvanized steel                                    6              Zincrometal coated steel                                       ______________________________________                                         The weldability of the coupons of Examples 5-7 was evaluated, and the     results of the evaluation are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                 Lobe     Nominal Location                                                                           Presence of                                             Width    of Lobe      Dynamic Resistance                             Example No.                                                                            (Amps.)  (Amps.)      Beta Peak                                      ______________________________________                                        4        2000     7700         Yes                                                              (min.)                                                      5        3000     8700         Yes                                                              (central)                                                   6        2400     8700         No                                                               (min.)                                                      ______________________________________                                    

The above-described examples demonstrate the improvement in resistancewelding characteristics which is obtained following the procedure of thepresent invention. The welding current required on the galvanized sheetsdecreased substantially and the lobe width increased significantlyfollowing application of the metal phosphide-containing coating. Inaddition, the presence of a beta peak was also detected afterapplication of the metal phosphide-containing coating. When a highelectrically resistant layer is present, such as for a Zincrometalcoating, the dynamic resistance beta peak does not occur, thus adverselyaffecting dynamic resistance feedback control.

EXAMPLES 7-24

Ferrophos-containing coatings were applied to bare steel test panels,hot-dipped galvanized steel test panels, and hot-dipped galvanized steeltest panels that had a conversion coating. The conversion coatingsemployed were Bonderite 37, a zinc phosphate coating, and Bonderite1303, a complex oxide coating. The binder was an epoxy ester resin, andthe coating was spray-applied to a thickness of 0.4 mils on both sidesof the test coupon.

The coated test panels were subjected to resistance welding tests todetermine whether they would weld. The welding conditions used weresimilar to those of Example 1, and the results are summarized in Table3.

                  TABLE 3                                                         ______________________________________                                                           Amount of                                                  Example                                                                              Surface     Ferrophos                                                  No.    Treatment   (wt. %)          Weldable                                  ______________________________________                                         7     None        None (Control, no binder)                                                                      Yes                                        8     None        0 (Binder only)  No                                         9     None        45               Yes                                       10     None        61               Yes                                       11     None        76               Yes                                       12     None        86               Yes                                       13     Bonderite 37                                                                              None (Control, no binder)                                                                      No                                        14     Bonderite 37                                                                              0 (Binder only)  No                                        15     Bonderite 37                                                                              45               No                                        16     Bonderite 37                                                                              61               No                                        17     Bonderite 37                                                                              76               Yes                                       18     Bonderite 37                                                                              86               Yes                                       19     Bonderite 1303                                                                            None (Control, no binder)                                                                      Yes                                       20     Bonderite 1303                                                                            0 (Binder only)  Yes                                       21     Bonderite 1303                                                                            45               Yes                                       22     Bonderite 1303                                                                            61               Yes                                       23     Bonderite 1303                                                                            76               Yes                                       24     Bonderite 1303                                                                            86               Yes                                       ______________________________________                                    

The results shown in Table 3 demonstrate the effectiveness in terms ofweldability of Ferrophos-containing coatings applied to galvanized steelwith or without conversion coatings at various pigment to binderloadings.

EXAMPLE 25

A Ferrophos-containing coating having a removable gelatin-based binderand containing about 86% by weight of pigment was applied to hot-dippedgalvanized steel test panels to a thickness of 0.5 to 1.0 mils. Afterdrying, the panels were resistance welded. The welding proceedednormally, and after cooling the panels were subjected to an aqueous washwhich effectively removed the remaining coating, leaving the surfacesuitable for subsequent finishing.

EXAMPLES 26-30

Ferrophos-containing coatings were prepared to evaluate the use of tin,lead and aluminum in combination with the Ferrophos pigment. TheFerrophos pigment used was Ferrophos grade HRS 2132, having an averageparticle size of about 3.0 microns and available from OccidentalChemical Corporation.

Three pigment compositions were prepared by grinding 15 grams of eitherthe tin, lead or aluminum powder with 500 grams of Ferrophos in a ballmill for 16 hours. The tin powder used was MD 301, available from AlcoaAluminum Co., the lead powder used was obtained from Fisher ScientificCo., and the aluminum powder usedwwas from Matheson, Coleman and Bell. Afourth pigment used was 15 grams of tin powder which was added to 500grams of Ferrophos without grinding. A control pigment was used withFerrophos alone, subject to the same milling, for comparison purposes.

Coatings were prepared using 200 grams of each pigment listed above, 30grams of an epoxy ester resin (Reichhold Epotuf 38-4071), 2 grams offumed silica (CabO-Sil), 1 gram of hydrophobic fumed silica (AerosilR972), and 0.1 gram of cobalt naphthenate. The solvent used was xylene.

The coatings were spray-applied to 4"×12"hot-dipped galvanized testpanels, aged, and subjected to testing. lest strips for each caatingwere all successfully resistance welded.

Although the present invention has been described with respect toseveral illustrative embodiments, it should not be interpreted as beingso limited. As will be evident to those skilled in the art, othersubstitutions and equivalents are possible without departing from thespirit of the invention or the scope of the claims.

What is claimed is:
 1. An article having improved resistance weldingcharacteristics, said article consisting essentially of:(a) a steelsubstrate, (b) a zinc metal or zinc alloy base layer applied directly tothe steel substrate by contacting the substrate with molten zinc or azinc alloy, or by electrodepositing the zinc or zinc alloy onto thesubstrate, and (c) a coating applied to said base layer, said coatingcomprising a binder and a pigment, said pigment consisting essentiallyof a least one metal phssphide selected from the group consisting ofphosphides of iron, nickel, cobalt, tin, copper, titanium, manganese,molybdenum, tungsten, vanadium, tantalum and mixtures thereof.
 2. Thearticle of claim 1 wherein the metal phosphide is ferrophosphorus. 3.The article of claim 2 wherein the coating has a non-volatile contentcomprising about 5 to about 96 percent by weight of binder and fromabout 4 to about 95 percent by weight of pigment.
 4. The article ofclaim 3 wherein the coating has a non-volatile content comprising about10 to about 70 percent by weight of binder and from about 30 to about 90percent by weight of pigment.
 5. The article of claim 2 wherein thepigment also contains up to abut 40% by weight of particles of a metalselected from the group consisting of tin, aluminum, lead, and mixturesthereof.
 6. The article of claim 5 wherein the metal particles have anaverge size within the range of about 0.1 to about 30 microns.
 7. Thearticle of claim 2 wherein the binder is readily removable.
 8. Thearticle of claim 2 wherein the pigment comprises particles having anaverage size within the range of about 0.1 to about 30 microns.
 9. Thearticle of claim 2 wherein the ferrophosphorus particles are coated witha layer of a metal selected from the group consisting of tin, aluminumand lead.
 10. The article of claim 1 wherein the coating has a thicknessof from about 0.1 mils to about 10 mils.
 11. The article of claim 1which has intermediate conversion coating between the zinc or zinc alloybase layer and the metal phosphide-containing coating.